Generally, automatic transmissions include a transmission control unit (TCU) for automatically controlling shift ratios according to variations in vehicle speed and load.
The TCU controls friction elements, a plurality of which are provided in a powertrain, to ON and OFF states to select one of the three elements of a planetary gearset (sun gear, ring gear, and planet carrier) to be an input element, another a reaction element, and the last an output element, thereby controlling the number of output revolutions.
To design a powertrain so that four forward speeds and one reverse speed can be output, one compound planetary gearset, comprised of two simple planetary gearsets, and at least five friction elements are used. The compound planetary gearset is structured such that elements from one planetary gearset are integrated or shared with elements from another planetary gearset.
However, the powertrain structured as in the above has the disadvantage of having a limited number of forward speeds, i.e., four. As a result, the powertrain can not make full use of the power provided by increasingly high-output engines. Also, the powertrain is not fuel efficient.
As a solution, there is provided a prior art powertrain having five forward speeds and one reverse speed. Such a powertrain is comprised of three simple planetary gearsets and at least six friction elements.
However, in such a powertrain for a 5-speed automatic transmission, to drive the vehicle in fourth or fifth-speed overdrive, there is a need for an element which rotates at a higher speed than that of a final output element. This results in a substantial amount of power loss. In addition, the large number of friction elements results in a heavy and large-size automatic transmission.